Optical metrology systems, for example, a opto-acoustic metrology system, can measure changes in reflectivity of the specimen that are due to a piezoreflective effect caused by ultrasonic waves propagating within the specimen. It can also measure the deflection of a laser beam due to the deformation caused by the ultrasonic waves. This type of metrology system, as well as other types known to those skilled in the art, use sensors that use position sensitive detection. These sensors detect a position and/or an intensity of a light spot in one or two-dimensions on a sensor surface. These sensors, referred to as “PSD sensors” may be either isotropic or discrete sensors. Further, PSD sensors may have one or more sensitive regions or cells.
In some opaque film metrology embodiments, multiple PSD sensors are used to measure film thickness. One such metrology system is the opto-acoustic system whose basic function is described in U.S. Pat. No. 5,748,318, the subject matter of which is incorporated by reference herein in its entirety.
In general, the opto-acoustic system measures the change in reflectivity (ΔR) of a specimen that is caused by the piezoreflective effect. The piezoreflective effect is modified by detect ultrasonic waves that travel within the specimen. See FIG. 1.
As shown, a pump beam 10 is directed at the surface 16 of the sample 14. The pump beam 10 causes rapid expansion in the surface 16 as shown by bump 15. This rapid expansion creates a shock or stress wave that is dispersed into the sample 14. The initial stress wave and the components thereof that return to the surface 16 of the sample 14 cause a change in the reflectivity of the surface 16 sample 14. A probe beam 12 is directed onto and reflected from sample 14. The reflected probe beam is incident upon a sensor 20 such as a PSD sensor that detects changes in the reflectivity of the surface based on changes in the reflected probe beam.
In addition, the system shown in FIG. 1 can also measure the deflection of a probe beam 12 that is caused by the same stress waves. As can be appreciated from the nature of the bump 15, as stress waves propagating within the sample 16 intersect the surface 16, they can cause a deformation in the surface 16. A split-cell detector 20 may be used to measure the deflection in the probe beam 12 caused by deformations in the surface 16 by measuring an imbalance in the incident probe power between the two cells of detector. See FIG. 2.
Typical PSD style sensors output two types of information. The first type is position sensitive information which relates a position of an incident spot of light (e.g. probe beam laser). As perturbations in the sample surface may deflect a probe beam incident upon the surface of a sensor, deflections of the probe beam may provide information about certain characteristics of the sample. The second type of information relates to the overall radiant power or flux incident upon the sensor. The amount of power incident upon the active surface of a sensor may provide information about certain characteristics of the sample. Therefore, taken alone or together and/or as a function of some other variable such as time, the types of information provided by PSD style sensors can be used to determine characteristics of the sample under test.
PSD style sensors and the metrology systems that use them must be manually aligned and maintained. Accordingly, it can be quite time consuming to modify the operation of a metrology tool to monitor a new product or feature. Where manual modification of sensor settings is required, it is often the case that these settings are not modified, thereby preventing one from discovering new applications for the metrology tool or making it difficult to truly dial in the sensitivity of a metrology tool to obtain optimal results.
Accordingly, there is a need for an automatically adjustable sensor mechanism for use in opto-acoustic metrology or other types of metrology operations.